The dualistic role of GTP on fat cell adenylate cyclase has been established. Low concentrations (less that 20 nM) of GTP activate the enzyme, while higher concentrations cause progressive inhibition, yielding a biphasic GTP titration. The biphasic behaviour is independent of assay conditions, such as time, temperature and metal ion concentration. Cholera toxin treatment of membranes or trypsin treatment of intact cells results in the abolition of the GTP inhibitory phase, while mercurial treatment causes the selective loss of the GTP activatory phase. Treatments which result in the loss of GTP inhibition also display no sensitivity to inhibition by adenosine or its purine-modified analogs. Such studies suggest that separate processes mediate the biphasic response of fat cell membranes to GTP: the function of the activatory phase is to amplify the stimulatory effects of hormones; the function of the inhibitory process is to amplify the inhibitory effects of adenosine. Preliminary results indicate that monavalent cations (Na+ greater than K+ greater than Li+) greatly enhance the inhibitory actions of adenosine.